1. Field of the Invention
This invention relates to multilayer structures, and more particularly to multilayer structures with selectable, rapidly propagating reaction wave fronts, as well as selectable total energies, adiabatic temperatures, ignition temperatures and ignition powers.
2. Description of the Related Art
Multilayer structures are thin-film materials that are periodic in one dimension in composition or in composition and structure. Composition/structure variation is generated during the synthesis of the structure, which is typically accomplished using atom by atom, atom by molecule, or molecule by molecule technologies. Individual component layers in a multilayer may vary in thickness from one atomic layer (.about.2 Angstroms) to thousands of atomic layers (&gt;10,000 Angstroms) of a given material. Multilayer structures can be synthesized using elemental, alloy, or compound layers to form both microstructures and combinations of elements/materials that cannot be produced using traditional processing technology.
Multilayers are made by alternate deposition of two or more different materials. After the first few layers, the structure of all the layers of one material are the same. The structure of each material is clearly of importance for the properties of the multilayer, not only in itself, but also for the influence it can have on the structure of the other material. Each material acts as a substrate for the deposition of the other.
The simplest multilayer structures are those which consist of a composition modulation imposed on a single structure. In almost all cases of this type, intermixing can lead to a uniform single phase of the starting crystal structure or atomic geometry.
While compositionally modulated multilayers may be regarded as a single phase, there are examples of two phase multilayers, in which the two materials have different structures and in which simple homogenization is not possible. If two phase multilayers are annealed, one material may diffuse in the other or react with it to yield a third phase. Alternatively, the two materials may be stable in contact with each other.
Whatever type of multilayer structure, the nature of the interfaces is of great significance. The atomic structure and the volume density of the interfaces between alternate layers in a multilayer can control or strongly affect the physical properties of the materials. In particular with regard to power dissipation of a multilayer structure during exothermic mixing of the alternating unreacted layers, both the number per unit volume and atomic structure of the interfaces control the rate at which the alternating elements mix and produce heat. The interface number per unit volume (density) can be controlled by varying the size of the period. The smaller the period, the closer the interfaces to each other and the higher their density. The atomic structure of the interface can be controlled by varying deposition parameters and/or deposition techniques.
Although multilayer structures can be found in equilibrium in natural systems, e.g., dichalcogenides, most artificial metallic multilayers have free energies far in excess of equilibrium and are susceptible to some type of transformation if there is sufficient atomic mobility. Contributing to the excess free energy are the interfacial free energy, the strain energies and excess chemical energy relative to a mixed composition. Stability is clearly important if the special properties of multilayers are to be exploited as deposited.
The simplest type of structural change in a multilayer is diffusional mixing at the interfaces. The increased interfacial diffuseness and the reduced amplitude of the composition modulation may affect many properties. The repeat distance of a multilayer period can also change.
Individual layer materials may show changes in structure. Crystalline layers may amorphize and amorphous layers may crystallize. The amorphorization and crystallization temperatures may be raised or lowered by interactions with the surrounding layers. Polycrystalline layers and mosaic layers may show grain growth. The grain boundaries running perpendicular to the layers are paths for fast diffusion, and they can enhance diffusional mixing and help destroy a compositional modulation.
A further type of structural change is reaction between the materials of the multilayer to give one or more new phases. If the multilayer is composed of elements with a strongly negative enthalpy of mixing, the heat released when the reaction is started with a thermal probe may be sufficient to allow it to proceed unassisted. This has been observed in transition metal/amorphous silicon multilayers. The phase which forms by reaction in a multilayer may itself be metastable.
The possible origins of the distinctive properties of multilayers are, (i) thin film effects, due to the limited thickness of one or more of the layers, (ii) interface effects, arising from the interactions between neighboring layers, (iii) coupling effects between layers of the same type, acting through the intervening layers and (iv) periodicity effects from the overall periodicity of the multilayer.
Multilayer properties can be tailored by controlling the period and structure of the alternate layers. The characteristics of the multilayer which may affect the properties are, (i) layer thickness and its spread (either periodic or non-periodic designs may be desirable), (ii) interfacial structure, including coherency, (iii) the crystal structure and crystallographic orientation (or amorphicity) of the constituent materials, (iii) the grain size in crystalline layers and (iv) the stresses in the layers.
The synthesis of multilayer structures can be accomplished by using techniques in which the product is formed by means of atom by atom processes. Such techniques include physical vapor deposition, chemical vapor deposition, electrochemical deposition, electrolytic deposition, atomic layer epitaxy and in some cases mechanical processing.
Multi-vapor-source configurations are used in the synthesis of metal multilayers with thermal sources. These are directly analogous to molecular beam epitaxy systems except that the sources need not be the Knudsen cell type. In these systems the sources and samples are stationary, the layering is achieved through interruption of the vapor streams to the substrate by the use of a rotating pin wheel or reciprocating shutters. Substrates can be held at temperatures from 4 to about 1300 degrees K. Heating mechanisms include electron beam bombardment and resistive and optical heating. Sample sizes are usually less than 25 cm.sup.2 and are dictated by specific system geometries and heating requirements.
Multisource configurations are also used in sputter deposition systems. In these systems the sputter sources are widely separated and the substrates moved past the sources, a single layer being deposited on each pass by a source. Sputter sources are solid materials, atoms or atom clusters being ejected from the solid target into the vapor by bombardment of the target surface with energetic particles. The ejected atoms impinge on a substrate and condense to form a film. In most cases, noble gases are used as the sputter gas, their ions being positively charged. The process is called cathodic sputtering. Ions are formed by establishing a plasma in much the same manner as a glow discharge is formed in a low pressure gas by an electric field between two electrodes. Factors to be considered include sputter source deposition surface coupling, the energy distribution of the sputtered atoms and the geometry of the vapor source substrate configuration.
The sputtering process entails establishing a plasma discharge and imposing a potential of the correct polarity so that ionized gas atoms are accelerated to the cathode surface, where, if of sufficient energy, they dislodge other atoms. These secondary atoms travel from the cathode surface to the deposition surface, being adsorbed to form a deposit.
There has been a limited understanding on the nature of interfacial interactions and on their relationship to the advance of the reaction front resulting from a chemical reaction between layers in the structure. The use of thin foils to investigate the propagation of such a combustion or reaction wave has been demonstrated for a nickel-aluminum system. Initiation of a reaction wave has been found to be triggered by the melting of nickel for a large period structure regardless of the composition of the foil, U. Anselmi-Tamburini and A. Z. Munir, J. Appl. Phys. 66 (10), pp 5039-5045, 1989. Additionally, the combustion synthesis of multilayer nickel-aluminum systems has also been reported by T. S. Dyer and Z. A. Munir, Scripta Metallurgica et Materialia, Vol. 30, No. 10 pp 1281-1286, 1994. However, these investigators have not produced multilayer structures with selectable wavefronts.
Accordingly, there is a need for a multilayer structure that has a selectable chemical reaction wavefront, a selectable initiation temperature by an external energy source and a selectable amount of energy delivered by a reaction of the alternating layers of the multilayer structure. It would be an advantage to provide multilayer structures in which one is able to determine the velocity of the chemical reaction wavefront, the total energy release, the rate of energy release, the adiabatic temperature and the ignitition temperature or power for such a wavefront. For a rapid heat source, there is a need to know how fast the wavefront travels which determines the rate at which energy is released by the structure. It would be desirable to provide multilayer structures that can be tailored for different applications depending on their chemical composition and physical structure that control their chemical reaction wavefronts.